If you import an assets file, say a weapon or playermodel, could you edit it?

I have seen some models having cut pieces, so I guess thats ok,

But how bout editing their structures, such as points or elongating?

I thought that, as long as you use it just for the original game you took it from, you can edit it as you like. (as long as you don't ask money for your services, which I don't think you were going to do )